Diabetes and Advanced Glycoxidation End Products AMY G. HUEBSCHMANN, MD 1 JUDITH G. REGENSTEINER, PHD 1,2 HELEN VLASSARA, MD 3 JANE E.B. REUSCH, MD 4,5 T he morbidity caused by diabetes has traditionally been classified into macro- and microvascular compli- cations. Although macrovascular compli- cations have received greater attention, microvascular complications are unique to diabetes, and hyperglycemia contrib- utes to their development. Numerous hy- perglycemia-related mechanisms are hypothesized to mediate micro- and ma- crovascular complications. These include the aldose reductase–mediated polyol pathway, the hexosamine pathway, pro- tein kinase C activation, generation of re- active oxidant stress, poly(ADP ribose) polymerase (PARP) activation, and accu- mulation of advanced glycoxidation (also termed advanced glycation or glycosyla- tion) end products (AGEs) (1,2). AGEs are particularly important, as they form intra- and extracellularly (3,4), are im- ported from food (5–9) and tobacco smoke (10), and can be deleterious, inde- pendent of hyperglycemia (9,11–16). They are implicated in the development of macrovascular disease (13,14,17–20), nephropathy (21–30), neuropathy (31,32), and retinopathy (21,33–38). The remediation of AGEs has also been shown to improve diabetic micro- and macrovas- cular disease (39 – 44). AGEs thus offer an important target for prevention of dia- betic morbidity. The focus of this review will be on the origin of AGEs, their mech- anism of injury, and therapeutic options under development. FORMATION OF AGEs — AGEs are nonenzymatically formed by reducing glucose, lipids, and/or certain amino ac- ids on proteins, lipids, and nucleic acids (Fig. 1A). For example, glucose and a free amino group form reversible intermedi- ates of a Schiff base and an Amadori prod- uct (e.g., HbA 1c ) before a series of reactions that irreversibly generate an AGE (45,46). This process was first iden- tified in 1912 and is known as the Mail- lard or “browning” reaction due to the associated yellow-brown color change (45,47,48). When formed endogenously, this reaction is driven forward by hyper- glycemia (4,49). Alternate mechanisms of AGE forma- tion include the “carbonyl stress” path- way, where oxidation of sugars and/or lipids create dicarbonyl intermediate compounds that use highly reactive car- bonyl groups to bind amino acids and form AGEs (50,51) (Fig. 1). Non– glucose-dependent AGE pathways in- volve neutrophils, monocytes, and macrophages, which, upon inflammatory stimulation, produce myeloperoxidase and NADPH oxidase enzymes that induce AGE formation by oxidizing amino acids (52,53). Once bound by AGEs, receptors for AGE (RAGE) associated with reactive oxygen species (ROS) generation pro- mote more AGEs via the NADPH oxidase pathway (54,55). Monocytes, macro- phages, and dendritic cells also secrete the nuclear protein amphoterin (also termed high-mobility group box 1 [HMGB1]) (56 –58), and HMGB1 can bind and activate RAGE and thus induce further inflammation (59 – 61). Another mechanism of AGE formation is the al- dose reductase–mediated polyol path- way. Glucose entering the polyol pathway may directly form AGEs via 3-deoxyglu- cosone AGE intermediates, but this reac- tion also causes depletion of NADPH and glutathione, and the resultant oxidative stress indirectly increases formation of AGEs (62). Given their varied mechanisms of for- mation, it is not surprising that AGEs are a heterogeneous group of compounds. Many AGEs fluoresce under ultraviolet light, and some are capable of intra- and intermolecular cross-linking, but not all share those properties (54,63). Once formed, certain cross-linking AGEs form stable cross-link structures with other proteins in the body, including structural proteins (e.g., collagen), intracellular pro- teins, membrane phospholipids, DNA, and lipoproteins (e.g., LDL cholesterol), and also bind to AGE receptors (64 – 67). ENDOGENOUS SOURCES OF AGEs IN DIABETIC SUBJECTS — People with diabetes have higher levels of AGEs than nondia- betic subjects because hyperglycemia and oxidative stress both contribute to their accumulation. Studies have shown 20 – 30% higher AGE levels in people with un- complicated diabetes (68,69) and 40 – 100% higher levels in subjects with type 2 diabetes complicated by coronary artery disease or microalbuminuria (17,70). Multivariate analyses in subjects with di- abetes have identified renal function, age, urinary albumin-to-creatinine ratio, sys- tolic blood pressure, and anemia as inde- pendent predictors of AGE levels (70,71). Renal impairment decreases clearance of AGEs in both diabetic and nondiabetic populations (51). Subjects with end-stage renal disease have shown significant ele- vations in circulating AGEs compared with healthy control subjects (by 5- to 100-fold) (46,72,73). Renal transplant has been shown to normalize AGE levels in subjects with end-stage renal disease (n = 2) (73). These observations indicate that AGE turnover is more dynamic than ●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●●● From the 1 Division of General Internal Medicine, Department of Medicine, University of Colorado Denver and Health Sciences Center, Denver, Colorado; the 2 Division of Cardiology, Department of Medicine, University of Colorado Denver and Health Sciences Center, Denver, Colorado; the 3 Division of Experimental Diabetes and Aging, Department of Geriatrics, Mount Sinai School of Medicine, New York, New York; the 4 Division of Endocrinology, Department of Medicine, University of Colorado Denver and Health Sciences Center, Denver, Colorado; and the 5 Denver VA Medical Center, Denver, Colorado. Address correspondence and reprint requests to Amy G. Huebschmann, MD, Assistant Professor of Medicine, University of Colorado Denver and Health Sciences Center, P.O. Box 6510, Mailstop F-729, Aurora, CO 80045. E-mail: amy.huebschmann@uchsc.edu. Received for publication 31 October 2005 and accepted in revised form 19 February 2006. Abbreviations: AGE, advanced glycoxidation end product; ARB, angiotensin-II receptor blocker; CML, N ε- carboxymethyllysine; HMGB1, high-mobility group box 1; NF-B, nuclear factor-B; PARP, poly(ADP ribose) polymerase; RAGE, receptors for AGE; ROS, reactive oxygen species. A table elsewhere in this issue shows conventional and Syste `me International (SI) units and conversion factors for many substances. DOI: 10.2337/dc05-2096 © 2006 by the American Diabetes Association. Reviews/Commentaries/ADA Statements R E V I E W A R T I C L E 1420 DIABETES CARE, VOLUME 29, NUMBER 6, JUNE 2006